Field of the Invention
The present invention relates generally to the field of vibration testing of objects such as satellites, instrumentation or any other object whose reliability in operation may be evaluated using high intensity vibration testing. Specifically, the present invention relates to means and methods for reducing deviations from the specified acoustic field due to constructive or destructive interference between multiple acoustic sources, acoustic standing waves or other modal acoustic behavior encountered in direct field acoustic testing.
Background of the Invention
The specification of co-pending U.S. application Ser. No. 13/117,870, filed May 27, 2011 titled Direct Field Acoustic Testing System and Method (hereinafter “the '870 application”) is incorporated by reference herein. As discussed in the '870 application, in the field of Direct Field Acoustic Testing (DFAT) it is generally desirable to obtain an acoustic field having a uniform spectral content and low coherence throughout the space around the Unit Under Test (UUT). As demonstrated in the '870 application, excellent spectral uniformity and low coherence was obtained at the control microphone locations through the use of a Multiple-Input-Multiple-Output (MIMO) arrangement incorporating multiple groups of independently controllable acoustic transducers. As discussed in U.S. Provisional Application No. 61/552,081 and corresponding co-pending International Application No. PCT/US12/62255 claiming priority thereto, both titled Drive Signal Distribution for Direct Field Acoustic Testing (hereinafter referred to collectively as “the Drive Signal Distribution applications”), both of which are incorporated by reference herein in their entirety, improved spectral uniformity at non-control microphone locations was obtained by distribution of combinations of drive signals to the groups of independently controllable acoustic transducers. However, more detailed measurements of the acoustic field around the UUT at non-control microphone locations have demonstrated the presence of significant deviations from uniformity at certain specific frequencies. It is believed that these deviations result either from the constructive or destructive interference of the output of multiple acoustic sources or from acoustic standing waves related to the geometry of the direct field acoustic test setup and the size shape and positioning of the UUT. Such deviations from spectral uniformity may result in over or under excitation of portions of the UUT. Accordingly it is desirable to provide methods for reducing said deviations by suppressing the acoustic mechanisms responsible for causing them.